1. Field of the Invention
This invention relates to a method of recovering hydrocarbons from a subterranean hydrocarbon-bearing formation, more particularly, this invention relates to a method of recovering hydrocarbons in which in situ combustion is first conducted in the formation after which a combustible gas and a combustion-supporting gas are injected into the formation in, for example, two separate intervals and oil is recovered from the formation.
2. Prior Art and Background
In recovery of petroleum from subterranean reservoirs, it usually is possible to recover only a minor portion of the petroleum in place by the so-called primary recovery techniques, that is, those techniques which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been developed in order to increase the recovery of petroleum in such reservoirs. In these supplemental techniques which are commonly referred to as secondary recovery operations, although they may be tertiary in sequence of employment, energy is supplied to the reservoir as a means of moving the fluid within the reservoir to suitable production wells through which they may be withdrawn to the surface of the earth.
Secondary recovery techniques which are showing increasing promise are those which involve in situ combustion. In an in situ combustion process, a portion of the carbonaceous material within the reservoir is burned or oxidized in situ in order to establish a combustion front. The combustion front may be moved through the reservoir by either a direct or inverse drive. In a direct in situ combustion process the combustion is initiated adjacent to one or more injection wells and the resulting combustion front is advanced through the reservoir in the direction of one or more production wells by the introduction of a combustion-supporting gas through the injection well or wells. The combustion front is preceded by a high temperature zone commonly called a retort zone, within which the reservoir petroleum is heated to affect a viscosity reduction and is subjected to distillation and cracking. The hydrocarbon fluids resulting from subjecting the petroleum to such a process are displaced to the production wells where they are withdrawn to the surface of the earth. In an inverse combustion drive, the combustion front is established adjacent to the production well or wells. As the combustion supporting gas is introduced through the injection well, the combustion front advances counter-currently to the flow of such gas in the direction of the injection well. The in situ combustion procedure, whether inverse or direct, is particularly useful in the recovery of thick heavy oils (petroleum) such as viscous petroleum crude oils and the heavy tar-like hydrocarbons present in tar sands. While these tar-like hydrocarbons may exist within the reservoir in a solid or semi-solid state, they undergo a sharp viscosity reduction upon heating and in an in situ combustion process they behave somewhat like the more conventional petroleum crude oil. In situ combustion also may be employed in the recovery of hydrocarbons from oil shale.